Tuesday, July 12, 2011

Neptune Discovered a Year Ago Today

At an average distance of 2.8 billion miles (4.5 billion kilometers) from the sun, Neptune takes about 165 Earth years to finish a single orbit.

One Neptunian year, that is, which is about 165 Earth years long.

That means July 12 marks the planet's first complete orbit around the sun since astronomers first observed it in 1846. The planet officially reaches this milestone at 22:27 UT.

Technically, Neptune's existence was mathematically predicted years earlier, based on the gravitational effect the ice giant planet was having on neighboring Uranus.

By looking for discrepancies in Uranus's orbit, Englishman John Couch Adams and Frenchman Urbain Le Verrier each independently calculated the unseen world's mass and position.

But Neptune's existence wasn't confirmed until September 23, 1846, when German astronomer Johann Galle used a telescope to search the predicted location in the sky and spotted a tiny blue-green disk. The planet was eventually named Neptune, after the Roman god of the sea.

Ironically, Galileo Galilei spotted Neptune more than 200 years earlier but wrongly assumed the planet was just a star.

"He observed it and he thought it moved relative to background stars, but it became cloudy and he was unable to observe it further," said Rocky Alvey, director of the Vanderbilt Dyer Observatory in Brentwood, Tennessee.

"If it hadn't been for clouds, Galileo may have been the discoverer of Neptune."